A printing device, known in the art as a “piezo-DOD inkjet printer”, generally includes a print head, which is arranged on a carriage which can move to and fro transversely with respect to the direction of movement of the substrate which is to be printed. The print head includes at least one spray nozzle, generally a number, for example 8 or 16, for each colour, this nozzle being in communication with a flexible working container via a feed passage. Furthermore, the print head for each spray nozzle includes a piezoelectric element for generating ink drops. A shockwave can be generated electrically in the print head by means of the piezoelectric element, with the result that each time each shockwave forms a drop from the printing medium. A drop of this type is only formed if it is required in order to print the substrate. This principle has been given the name drop-on-demand. In addition to “piezo-DOD”, “thermal DOD” is also known, in which heating elements are used instead of a piezoelectric element to form drops.
It is generally recognized in the art that degassed printing medium is required for piezo-DOD. If printing medium which has not been degassed (for conventional types of ink the equilibrium concentration of oxygen is approx. 8 mg of O2/l at atmospheric pressure) is used, at high firing frequencies the gas accumulates in the print head, where the accumulated gas attenuates the shockwaves which bring about the formation of drops. The result of this is that after a certain time has elapsed, fewer drops are formed, or even no more drops will be formed, and the inkjet printer starts to malfunction or even stops altogether. With printing medium which has not been degassed, this phenomenon means that it is necessary either to use lower firing frequencies or to periodically interrupt printing operation in order to remove the accumulated gas, generally by purging. Both solutions reduce the productivity of the printing device. If degassed ink is used, this phenomenon, and the associated adverse effects, do not occur. For example, degassed printing medium has an oxygen concentration of approximately 1 mg/l. If degassed printing medium is in contact with air for approximately one day, the oxygen concentration returns to the abovementioned equilibrium value. Therefore, contact with air has to be avoided, although brief contact is acceptable.
The degassing of the printing medium may be carried out on line during printing, for example just before the printing medium is supplied to the print head or in the print head itself. The latter option is used in particular in relatively large industrial printing devices. Another possibility is to use printing medium which has already been degassed, a solution which is adopted in particular for relatively small standard printing devices. This imposes higher demands on the packaging, in particular with regard to its gas and/or air permeability. Examples of packaging of this nature are described in European patent applications 857 573 and 1 013 449.
In a type of piezo-DOD printing device as described above, the working container, which is flexible (not dimensionally stable) with a view to allowing it to empty out without the admission of air, is arranged at a fixed position and connected to the print head by means of a flexible line of sufficient length, since allowing the working pouch to move with the print head, for example on the carriage, can lead to undesirable fluctuations in the pressure of the printing medium in the print head, and therefore to a non-uniform print quality. The known working container has a relatively small volume (of the order of magnitude of a few hundred millilitres), since the pressure in the print head is related to the pressure exerted by the printing medium (volume and working height with respect to the print head), and consequently frequent replacement by operating staff is required. The printing device has to be stopped for this purpose.
Furthermore printing devices are known in the art, which are provided with feed systems, mostly controlled by valves, for feeding a printing medium from a reservoir to a working container. Examples thereof are known from e.g. JP-A-11105299, EP-A-0 927 638 and JP-A-2003118134. The complexity, costs and susceptibility to disturbances of the printing device are increased by such systems.